I wouldn't say the demise of British brands, nor the near demise of H-D was caused by "bad engineering", per se.
Nope! I think a more valid reason would be the lack of reinvesting into future R&D which both entities were guilty of during their heyday of the 1960s, and as you mention, just as Honda and the rest of Japan Inc. were developing whole new modern and less "cantankerous" motorcycles for the public.
In other words, considering the basic design of both the Triumph vertical twins AND the H-D OHV V-Twins were debuted in the late 1930s, and basically over the years both of them were just given mild updates, and while they might've have been well engineered for 1930s/40s/50s/60s, they were eventually overtaken in many areas by Japan Inc, and the purchasing public recognized this by the early 1970s.
